1. Field of the Invention
The present invention relates to an improved process for the production of microcapsules by use of a novel combination of emulsifiers in a solution from which the microcapsules are formed. The process is particularly useful for the production of microcapsules containing a colorless dye intermediate used in carbonless copy paper (CCP) applications.
2. Description of the Prior Art
In the broadest sense, micro-encapsulation provides a means of packaging, separating and storing materials on a microscopic scale for later release under controlled conditions. Minute particles or droplets of almost any material may be encased by a capsule wall and thus isolated from reactive or otherwise hostile surroundings. The contents of a microcapsule may be made available by mechanical rupture of the capsule wall or its disintegration by chemical means.
The encapsulated product, the core, may be a single substance or a mixture, and may be liquid or solid in nature. The core may be, in itself, the primary material to be encapsulated, or it may be a vehicle containing the desired material in solution, emulsion or dispersion form.
The most commonly used wall materials are natural or synthetic polymers. The wall material is usually inert. A tougher, less permeable capsule wall may be formed by inclusion of an isocyanate in the wall composition. The isocyanate acts as a cross-linker between chains of polymer molecules, joining certain carbon atoms of the chains by the formation of chemical bonds.
Certain micro-encapsulation procedures are based upon interfacial polymerization reactions. Procedures based upon interfacial polymerization reactions make use of an emulsion in which each of a pair of immiscible liquids has dissolved in it a complementary reactant of a polymer-forming system. When the two reactants meet at an interface, polymerization proceeds and a thin, insoluble film forms, which in turn, isolates the core material from its surrounding environment. A dispersing agent may be used to promote uniformity and maximum stability of the dispersed phase.
Hydrogen transfer polymerization is an interfacial polymerization reaction in which the molecule undergoing polymerization rearranges to an energetically preferred structure so that the repeat unit of the polymer does not possess the structure of the original molecule. More specifically, it is a polymerization that proceeds by intermolecular hydrogen migrations. The characteristic feature of compounds that undergo hydrogen transfer polymerization is their possession of a labile hydrogen atom, i.e., a hydrogen atom bonded to an electronegative atom or to an atom activated by strongly electro-negative substituents.
Following capsule wall formation, the resultant microcapsule containing slurry may be spray-dried. A spray dryer is a large, usually vertical, chamber through which a hot gas is blown, and into which a liquid, slurry, or paste may be sprayed by a suitable atomizer. The size of the spray-dried particles may range from about 2 .mu.m to 1 mm in diameter. All drops must be dried until no longer sticky, before they strike the chamber wall. Spray dryers are particularly useful in their ability to handle certain materials due to the short contact time of the material in the dryer and in their ability to handle concentrated solutions. The viscosity of the solutions to be sprayed may be as high as 300 cP, which means that less water must be removed from concentrated solutions.
The production of carbonless copy paper, wherein a colorless dye intermediate such as, crystal violet lactone or benzoyl leuco methylene blue, is microencapsulated to form particles less than 20 .mu.m in diameter, is a major application of micro-encapsulation processes. The microcapsules are deposited in a thin layer on the underside of a sheet of paper in contact with a second receiving sheet of paper. The top side of the receiving sheet is sensitized with an acid reactant. The microcapsules are designed to resist breakage under normal conditions of storage and handling, but to break under the high local pressure of a pen or pencil point, or a typing element. When the microcapsules break, the dye intermediate is released so that it can contact the acid and react with it, thereby producing a colored image on the receiving sheet.
The preparation of microcapsules suitable for use in carbonless copy paper applications is well known in the art. Such a system was first disclosed in U.S. Pat. No. 2,712,507, issued July 5, 1955, on the application of Barrett K. Green. Green disclosed a process wherein a small amount of crystal violet lactone was dissolved in an oily fluid and enclosed in microcapsules whose walls ruptured upon high local pressure. The microcapsules were coated in a thin layer on the underside of a sheet of paper beneath which was a second sheet of paper sensitized with an acidic reactant. The encapsulated dye intermediate, subsequently transferred by printing pressure to the receiving sheet, assumed a visually distinctive color upon reaction with the acid. This system is now used throughout the world in the application of micro-encapsulation to the production of carbonless copy paper.
Microcapsules suitable for use in the production of carbonless copy paper must meet certain requirements. The capsule walls must be impermeable to the colorless dye intermediate and to its solvent. Permeability to the colorless dye intermediate causes discoloration; permeability to the solvent causes desiccation of the capsule's contents and thus reduces the capsule's effectiveness. The capsule walls should be as impermeable as possible to oxygen, to light, and to acids and bases.
The capsule walls must be sufficiently strong to provide microcapsules which break only under the high local pressure exerted by a writing intrument; the walls must be strong enough to resist other types of pressure.
Dry free flowing capsules having less agglomeration can be produced by spray-drying the microcapsules of the present process. Certain carbonless paper products require the incorporation of these capsules in hot melt waxes, inks, or similar type vehicles for spot coating applications. Smoother coatings are easier to produce because the free-flowing capsules are easier to disperse. Breakage of the capsules during coating and handling is reduced because the capsules are better dispersed and less agglomerated.
In order to form less permeable microcapsules, it is important that the proper emulsifying agent is chosen. Additionally, the effect of the emulsifier on the subsequent spray-drying characteristics of the microcapsules so formed must also be considered. It is well-known in the art that partially hydrolyzed polyvinyl alcohol (PVA) can be used as an emulsifier for micro-encapsulation processes. It has been found however, that such microcapsules are still undesirably permeable. Further, spray-drying of microcapsules obtained using PVA as an emulsifier is difficult since agglomeration of the microcapsules occurs.
Additionally, Becher et al., U.S. Pat. No. 4,563,212, describes a micro-encapsulation procedure based upon an interfacial polymerization reaction wherein the material to be encapsulated is an agricultural chemical such as an herbicide, an insecticide, a plant growth regulator or an herbicidal antidote.
Becher et al. disclose a process wherein a water-immiscible material, containing the first shell wall component, is emulsified into an aqueous solution containing an emulsifier selected from the group consisting of sulfonated napthalene formaldehyde condensates, sulfonated polystyrenes and functionalized oligomers. In Becher et al., an oil-in-water emulsion is formed with the aid of high shear; the second shell wall component is added to the oil-in-water emulsion, and after a short period of time, the shear rate is reduced. Shear is continued for varying periods of time, following which salt is added to the suspension to balance its density. The formulation is subsequently bottled.
It is taught that the use of these emulsifiers increases the amount of active ingredient which is encapsulated from solution compared to known emulsifiers. The microcapsules disclosed by Becher et al. undergo no further treatment such as separation from the aqueous liquid, but may be directly utilized or combined with liquid fertilizers, insecticides or the like to form aqueous solutions which may be conveniently applied in agricultural uses.
Thus, the prior art has not disclosed a process for the production of microcapsules suitable for use in carbonless copy paper applications which provides capsule walls as impermeable as would be desired to the colorless dye intermediate, and as strong as would be desired to withstand pressures other than the high local pressure exerted by a writing instrument. Nor does the prior art disclose an interfacial micro-encapsulation process providing adequately free-flowing capsular powder.